HD Pentax-DA 20-40mm f/2.8-4 Limited DC WR Lens Review

HD Pentax-DA 20-40mm F2.8-4 ED Limited DC WR Performance

At 20mm and maximum aperture, sharpness is outstanding in the centre of the frame, but the lens doesn't perform as well towards the edges of the frame, only producing fairly good sharpness. Stopping down to between f/5.6 and f/8 produces excellent sharpness across the frame at this focal length.

Zooming to 30mm results in a slight increase in sharpness across the frame at maximum aperture. Here clarity is outstanding in the centre, but falls just short of good sharpness towards the edges of the frame. Stopping down to between f/5.6 and f/11 results in excellent sharpness across the frame at this focal length.

Finally, at 40mm, sharpness is very good in the centre of the frame at f/4, but still only fairly good towards the edges of the frame. Again, stopping down improves performance greatly, with peak sharpness across the frame being realised at f/5.6.

MTF@20mm

MTF@30mm

MTF@40mm

How to read our charts The blue column represents readings from the centre of the picture frame at the various apertures and the green is from the edges. Averaging them out gives the red weighted column. The scale on the left side is an indication of actual image resolution. The taller the column, the better the lens performance. Simple. For this review, the lens was tested on a Pentax K-5 IIs using Imatest.

Chromatic aberrations are extremely well controlled, barely exceeding a quarter of a pixel width towards the edges of the frame throughout the zoom range. This level of fringing will be difficult to spot, even in harsh crops from the edges of the frame along high contrast edges.

CA@20mm

CA@30mm

CA@40mm

How to read our charts Chromatic aberration is the lens' inability to focus on the sensor or film all colours of visible light at the same point. Severe chromatic aberration gives a noticeable fringing or a halo effect around sharp edges within the picture. It can be cured in software. Apochromatic lenses have special lens elements (aspheric, extra-low dispersion etc) to minimize the problem, hence they usually cost more. For this review, the lens was tested on a Pentax K-5 IIs using Imatest.

Falloff of illumination is fairly typical for a standard zoom. At 20mm the corners of the frame are 1.25 stops darker than the image centre and at 40mm the corners are 1.21 stops darker. Visually uniform illumination is achieved with the lens stopped down by two stops from maximum aperture or beyond throughout the zoom range.

Distortion is also fairly typical for a lens of this type. Imatest detected 2.92% barrel distortion at 20mm and 0.627% barrel distortion at 40mm. Fortunately the distortion pattern is uniform across the frame, which should make it relatively easy to apply corrections in image editing software afterwards.

During testing, this lens proved itself very resistant to flare and contrast levels are good, even when shooting into the light. The shallow metal hood doesn't provide much shade for the front element, but is useful for preventing bashing the front of the lens, at least.

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